1. Field of the Invention
This invention relates to a hotmelt adhesive based on a compatible polymer mixture of a polyamide and an ethylene copolymer. The polyamide contains dimeric fatty acids, while the ethylene copolymer contains carboxylic acid anhydride.
2. Statement of the Related Art
Hotmelt adhesives are growing in popularity in many branches of industry. They afford the user the advantage that they reach the necessary bond strength simply by cooling of the melt and, accordingly, may be used in high-speed production processes. Another advantage lies in the fact that no solvents are used, so that energy is no longer required either for drying or for recovering the solvents, nor are any measures necessary for preventing pollution of the working environment.
Polyamides are an important class of hotmelt adhesives. Among polyamides, those based on dimeric fatty acids are particularly important because polyamides such as these on the one hand have a relatively narrow softening range and, on the other hand, show no tendency towards crystallization and resulting embrittlement, even at temperatures below 0.degree. C.
Although products such as these show excellent properties for numerous applications, efforts are still nevertheless being made to develop special hotmelt adhesives having even better adhesion properties. Thus, attempts have been made not only to modify the polyamides and to adapt them to the individual application envisaged by appropriate choice of the individual co-condensed units, but also to provide compatible polymer mixtures having special properties. In the present context, compatible polymer mixtures are understood to be mixtures which, in their solid state and up to the processing temperature above the melting point, appear to the eye to be homogeneous and of one phase. Physically, the polymer mixtures in question are either true solutions or finely divided dispersions of one polymer in the matrix of the other.
U.S. Pat. No. 4,018,733 describes compatible mixtures of: (a) an acidic copolymer based on ethylene having an acid number of from 3 to 80; (b) a polyamide having an amine number of from 70 to 400; and also other components; in an a:b weight ratio of 0.25-4:1. The polymer mixtures in question have adequate technical properties for solving numerous bonding problems. However, they are only suitable to a limited extent, if at all, for bonding extremely difficult materials, for example copper or non-pretreated polyethylene.